This invention relates to copying machines and particularly to apparatus for fixing copy paper for use in electrophotographic appliances, such as laser beam printers or copying machines.
In conventional electrophotographic appliances, toner is transferred to copy sheets by a photosensitive drum and the toned copy sheets are fed to a fixing roller. The copy sheets are required to be flattened at their leading edge when they pass the fixing roller. For this purpose, some fixing structures have been proposed.
One of the fixing structures comprises a heating roller having various diameters. That is, the diameter of the opposite ends of the roller are larger than that of the middle portion of the roller. With this construction, when copy sheets pass the fixing station, they come in contact with the heating roller irregularly, which makes their coefficient of linear expansion different. Accordingly, the copy sheets were frequently crumpled.
Another fixing structure includes a copy sheet guide having guide ribs. Of the guide ribs, the middle rib is higher than the end ribs, which prevents sagging of copy sheets at the leading edges thereof. Otherwise, a bias voltage having a polarity opposite to that of the toner on the copy sheets is applied from the copy sheet guide. With this construction, the attraction between the polarities enables the leading edges of the copy sheets to be flattened.
The above conventional fixing structure was of little use in feeding the copy sheets in a flattened and correct state.
It is economical to use both sides of copy sheets. For this purpose, copy sheets are required to be flattened at their leading edges after fixation.
In the conventional fixing device as shown in FIG. 5, which is used in laser beam printers or desk-type copying machines, the outside diameter of the heating roller 1 in contact with the pressure roller 3 is smaller than that of the pressure roller, whereby the leading edges of copy sheets 2 bow in the circumferential direction of the heating roller 1 when passing through a nip defined between adjacent peripheries of the rollers 1 and 3.
The discharge of the copy sheets in the bent state at their front edges cause the front edges to be caught in the fixing station when one side copied sheet re-passes the fixing station for copying on the other face. This also causes paper jams in the machine.
A conventional cleaning means 6 in contact with the heating roller 1 is fixed with respect to the heating roller 1. The foreign matters removed from the heating roller 1 are stacked below the cleaning means 6.
This conventional fixing station has no means for removing the foreign matters stacked below the cleaning means 6 and the stacked foreign matters damage the surface of the heating roller when passing the space between the cleaning means and the heating roller, thus decreasing the life of the fixing means. Furthermore, the face of the copy sheet having passed the fixing means becomes filthy, thus lowering fixing performance and quality of the copying device.